In recent years, in the field of display devices comprising display members such as large flat panel displays and small lightweight displays, reductions in the thickness of the structural members and the direct bonding of front surface optical members to the display members have resulting in ongoing reductions in the thickness of display devices as well as improved contrast. However, in those cases where a thin display is used as a display member, and an optical member is bonded directly to the front surface of the display member, stress caused by expansion and contraction of each member and residual stress from the bonding results in the generation of internal stress, which tends to cause display irregularities within the display. Accordingly, in recent years, there has been much investigation of resin layer materials that can be interposed between the display member and the optical member.
For example, Patent Document 1 (JP 2001-316447 A) discloses, as a resin layer material, a soft composition comprising a polyol having a functionality of 2.4 to 3.0 and a molecular weight of 3,000 to 6,000, and a secondary or tertiary higher monoalcohol that functions as a tackifier, wherein the composition exhibits pressure-sensitive adhesiveness and has a rubber hardness of 30 or less. Because this soft composition utilizes a urethane reaction during the curing reaction, the process requires a considerable length of time, with a reaction at 100° C. for 2 hours followed by additional curing for 7 days, and therefore the process is suited to cast molding. However, the composition is not suitable for production methods that are more efficient than cast molding, such as thick sheet production methods in which the composition is applied continuously, without incorporating gas bubbles, to a film that functions as a substrate.
Patent Document 2 (JP 2004-359808 A) discloses, as a resin layer material, a transparent gel pressure-sensitive adhesive and sheet that are prepared by swelling a three dimensional cross-linked polymer with a liquid comprising a plasticizer and inorganic particles. However, because this pressure-sensitive adhesive and sheet contain a plasticizer, they tend to be prone to bleeding of the plasticizer, which can stain or corrode the bonded item such as the display member.
Patent Document 3 (JP 2006-309114 A) discloses, as a resin layer material, a pressure-sensitive adhesive sheet having a storage elastic modulus G′ at a temperature of 23° C. and a frequency of 1 Hz within a range from 3×105 to 1×107 Pa. Considering the operating environment for a display device, the pressure-sensitive adhesive sheet preferably has an appropriate elastic modulus across a temperature range from 0° C. to 50° C. However, the above document makes no disclosure regarding the elastic modulus of the pressure-sensitive adhesive sheet under low-temperature conditions.
If the elastic modulus at low temperatures is high, then there is a possibility that when the display device is used in a low-temperature environment, the stress relaxation properties and the pressure-sensitive adhesiveness of the sheet may be unsatisfactory, thereby causing a problem. More specifically, if the above pressure-sensitive adhesive sheet is used for the display device of a mobile phone or the like, which tends to be more susceptible to air temperature effects, then the display tends to be prone to display irregularities in low-temperature environments, and detachment may occur at the pressure-sensitive adhesive sheet interface if the mobile phone is exposed to impact such as when the phone is dropped. Moreover, if the elastic modulus under high-temperature conditions of 50° C. or higher is too low, then strain within the various optical members cannot be satisfactorily suppressed, increasing the possibility that detachment or display irregularities or the like may occur.
In light of these circumstances, the inventors of the present invention have disclosed, in Patent Document 4 (JP 2008-56757 A), a sheet that uses a specific urethane (meth)acrylate oligomer composition. However, this sheet tends to suffer from a dramatic reduction in cohesive strength in high-temperature environments. As a result, in those cases where a plastic sheet such as an acrylic sheet is used as an optical member, thermal deformation of the plastic sheet may sometimes lead to lifting or detachment at the peripheral portions of the sheet.
As described above, although each of the disclosures for the resin layer material for a display device shows capabilities of improving display irregularities within the display device, none is capable of combining this improvement with requirements of mass productivity, anti-staining properties, low-temperature impact resistance and high-temperature reliability. Accordingly, a resin layer material that exhibits excellent stress relaxation properties and improves display irregularities, but also exhibits excellent anti-staining properties and has superior reliability under various operating conditions, as well as a method for producing such a resin layer material with good efficiency, have been keenly sought.